Folder for container template files

Sorry for bothering the community with a question, which should probably be obvious to a concrete user, but in my case it is not: where (in which folder?) should the container template files be placed in order to be recognized as not missing? In Elemental theme there are icons for variety of container types, but without files defining their layouts a user is unable to add them.
Please advise.

In a theme its at my_theme/elements/containers